The invention relates to an apparatus for effecting plethysmography, comprising a measuring cell adapted to enclose a person to be examined, and mainly consisting of a transparent material, a smaller reference cell constructed in such manner that exterior pressure variations cause equal pressure variations within both cells, and means for measuring the difference between the pressure in the two cells.
(B) Prior Art
Plethysmography is a method for measuring the volume variations of the thorax of a person to be examined during respiration. For this purose the person is enclosed in an airtight measuring cell, so that the volume variations of the thorax cause pressure variations within the measuring cell. These pressure variations may be measured, and may be used to determine the volume variations of the thorax with the aid of suitable tables. Of course, the pressure variations occurring in the measuring cell are extremely small, so that the measurement is strongly disturbed by pressure variations in the atmosphere outside the measuring cell, for instance due to the opening and closing of doors, vibrations, sounds, and the like.
It is known to reduce the influence of the exterior pressure variations by using a reference cell of smaller dimensions, which is constructed in such manner that the exterior pressure variations cause equal pressure variations in both cells, and by measuring the pressure difference between the two cells. In order to obtain an exact compensation of the exterior pressure variations, the reference cell must satisfy the condition that the ratio of the volume to the compliance must be equal to the corresponding ratio for the measuring cell. In this connection, the compliance of a cell may be defined as a positive value corresponding to the derivative of the volume as a function of the difference between the pressure outside and inside the cell.
It is usual to make the measuring cell as rigid as possible, so that the influence of the exterior pressure variations is reduced to a minimum. To satisfy this requirement, the measuring cell is generally made of an opaque material, and only provided with a small window in the wall to observe the patient. However, the person to be examined feels very uncomfortable in a cell of this kind, since he is locked up in a small space, and completely isolated from the surroundings. It has been proposed to remove this disadvantage by the use of a transparent synthetic material, such as polymethylmethacrylate, for the walls of the measuring cell, but expensive and complicated constructions have been necessary in this case to obtain a sufficient rigidity of the measuring cell, since the natural rigidity of the material is very small.
An apparatus for effecting plethysmography comprising an opaque measuring cell with a small window in the wall has been disclosed in Dutch Patent application No. 65,16124, laid open to public inspection on June 13, 1966.
An apparatus for effecting plethysmography comprising a measuring cell made of a transparent material whose rigidity has been increased by a special construction has been disclosed in an article by F. D. Stott, entitled "The Whole-body Plethysmograph", and published in the British periodical "Instrument Practice", Jan. 1963, pages 48-51.
Both above-mentioned publications also disclose a reference cell for compensating exterior pressure variations.